Power system for a hybrid fuel cell vehicle that employs a floating base load strategy

ABSTRACT

A fuel cell system employing a floating base load hybrid strategy for reducing fast voltage transients of a FCPM. A power request signal is applied to an average power calculation processor that calculates the average power requested over a predetermined previous period of time. A weighting function processor provides a weighting function based on the state of charge of an EESS. The power available from the FCPM and the EESS is applied to a power comparison processor. The available power is compared to the power request to provide a difference value between what is currently being provided and what is desired. The difference value is compared to power limit values of the EESS. The output value of this comparison is added to a filtered value to generate a signal for the change in the output power of the fuel cell stack based on the power request.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/313,162, filed Dec. 20, 2005 and titled “Floating Base LoadHybrid Strategy For A Hybrid Fuel Cell Vehicle To Increase TheDurability Of The Fuel Cell System.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a power system for a hybrid fuelcell vehicle and, more particularly, to a power system for a hybrid fuelcell vehicle that employs a floating base load strategy that reducesfast power transient demands from the fuel cell power module.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electro-chemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free hydrogen protons and electrons. The hydrogenprotons pass through the electrolyte to the cathode. The hydrogenprotons react with the oxygen and the electrons in the cathode togenerate water. The electrons from the anode cannot pass through theelectrolyte, and thus are directed through a load to perform work beforebeing sent to the cathode. The work acts to operate the vehicle.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. The fuel cell stack receives a cathode inputgas, typically a flow of air forced through the stack by a compressor.Not all of the oxygen is consumed by the stack and some of the air isoutput as a cathode exhaust gas that may include water as a stackby-product. The fuel cell stack also receives an anode hydrogen reactantgas that flows into the anode side of the stack.

Most fuel cell vehicles are hybrid vehicles that employ a rechargeableelectrical energy storage system (EESS) in combination with the fuelcell stack, such as a DC battery or a super-capacitor. The EESS providessupplemental power for the various vehicle auxiliary loads, for systemstart-up and during high power demands when the fuel cell stack isunable to provide the desired power. More particularly, the fuel cellstack provides power to an electric traction system (ETS) and othervehicle systems through a DC voltage bus line for vehicle operation. TheEESS provides supplemental power to the voltage bus line during thosetimes when additional power is needed beyond what the stack can provide,such as during heavy acceleration. For example, the fuel cell stack mayprovide 70 kW of power. However, vehicle acceleration may require 100 kWor more of power. Therefore, the EESS would provide the extra 30 kW ofpower. The fuel cell stack is used to recharge the EESS during thosetimes when the fuel cell stack is able to meet the system power demand.The generator power available from the ETS during regenerative brakingis also used to recharge the EESS through the DC bus line.

As discussed above, the power demand from the ETS can be provided by thefuel cell stack, the EESS, or a combination of both. Normally, the EESScan provide energy faster than the fuel cell stack, and therefore canalso increase the dynamic capabilities of the vehicle. Also, the fuelcell system can be made smaller and still provide the same drivingcapabilities, or the dynamic requirements of the fuel cell system can bereduced, which can increase durability.

For a typical hybrid vehicle strategy, the EESS is mainly used toincrease efficiency, to lower the dynamic requirements of the fuel cellsystem, and/or to increase the performance of the vehicle. If the ETSdemands more power, the EESS can provide the stored energy to the ETSvery fast. Additional demanded power can be quickly provided by the fuelcell system.

The fuel cell system power demand for certain vehicle drive cycles mayrequire that the fuel cell system operate in very different and fastchanging power levels with high power gradients. These frequent changesin power may cause many voltage changes in the stack output power thatreduces the lifetime and durability of the stack. In addition, fuel cellsystem components are highly stressed during hard power transients ofthe fuel cell stack. Therefore, a reduction of fast voltage changes willimprove the durability of the fuel cell stack.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a hybrid fuelcell system employing a floating base load strategy for reducing fastvoltage transients of a fuel cell system is disclosed. The power requestfrom an ETS is applied to an average power calculation processor thatcalculates the average power requested over a predetermined previousperiod of time. A state of charge signal is applied to a weightingfunction processor that provides a weighting function based on thecurrent state of charge of an EESS. Particularly, the weighting functionis a value that approaches zero as the state of charge of the EESSapproaches its maximum. The weighting function and the average powercalculation value are multiplied to generate a filtered base load demandto the fuel cell power system. The power available from the fuel cellstack and the power available from the EESS are applied to a powercomparison processor. The combined currently available power from thestack and the EESS is compared to the request for the ETS to provide adifference value between what is currently being provided and what isdesired. The difference value is added to the filtered value to generatea signal that changes the output power of the fuel cell stack for thecurrent power request.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a power system for reducing fast voltagetransients of a fuel cell power module, according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa power system for a hybrid fuel cell vehicle that reduces fast voltagetransients is merely exemplary in nature, and is in no way intended tolimit the invention or its applications or uses.

As will be discussed in detail below, the present invention provides asystem and method for reducing fast voltage transients of the outputpower of a fuel cell stack in a hybrid fuel cell vehicle. The vehicleincludes a hybrid power system having a fuel cell power module (FCPM)and an electrical energy storage system (EESS). The FCPM, the EESS, orboth, can provide the required power to the ETS depending on the driverpower command. The operation strategy of the invention determines thedesired fuel cell system power by considering the actual power demand,an average power demand for a certain preceding time period, the actualEESS power limits and the state of charge of the EESS.

FIG. 1 is a block diagram of a power system 10 for a fuel cell electrichybrid vehicle, according to an embodiment of the present invention. Thesystem 10 includes a FCPM 12 having a fuel cell stack, and an EESS 14,such as a DC battery, accumulator, super-capacitor or combinationthereof, that provides supplemental power. An average power calculationprocessor 16 receives a power demand signal on line 18 representative ofa power request for the ETS from the vehicle driver for a particularvehicle speed. The processor 16 calculates an average power demand overa preceding period of time, for example, a few seconds.

A signal of the current state of charge (SOC) of the EESS 14 is sent toa weighting function processor 20 on line 22. In one non-limitingembodiment, the weighting function has a minimum value of zero and arange that depends on predetermined battery parameters and the batteryoperation strategy. As the SOC of the EESS 14 approaches its maximumvalue, the weighting function is reduced to its minimum value so thatthe output power from the FCPM 12 used to recharge the EESS 14 isreduced. When the SOC of the EESS 14 is at its minimum value, then theweighting function is increased to its maximum value so that the outputpower from the FCPM 12 is increased to recharge the EESS 14. Thus, theweighting function changes the average power signal depending on the SOCof the EESS 14 so that the output power of the FCPM is high enough tosatisfy the power demand and the charging requirements of the EESS 14.

The average power and the weighting function value based on the SOC ofthe EESS 14 are multiplied by a multiplier 24 to provide a filtered baseload demand for the FCPM 12 that is averaged over a certain time periodto reduce the fast voltage transients. The multiplied value is then sentto an adder 26.

The power being currently provided by the FCPM 12, the current powerlimits of the EESS 14, including both an available charge signal and anavailable discharge signal, and the power demanded from the ETS by thevehicle operator are provided to a power comparison processor 28. Theprocessor 28 subtracts the power being currently provided by the FCPM 12from the power demand signal on the line 18. This difference representsthe power the EESS 14 has to provide. To ensure that the EESS 14 stayswithin its power limits, the difference signal is compared to thedischarge or charge limit of the EESS 14, depending on the sign of thedifference signal. Normally if the power difference stays within thepower limits of the EESS 14, the output signal of the comparisonprocessor 28 is zero. Thus, only the base load generated by themultiplier 24 commands the FCPM demand through the adder 26. If thepower difference is not within the power limits of the EESS 14,additional or less power will be immediately demanded from the FCPM 12by the comparison processor 28. The additional or less power is sent tothe adder 26 to add it to the floating base load to generate a demandsignal that changes the output power of the FCPM 12 to meet the driverdemand. The actual EESS power limits (discharge and charge limit)provided to the comparison processor 28 can be dependent on the size ofthe EESS 14, current technology, the temperature of the EESS 14, SOC ofthe EESS 14, age, etc.

Based on the discussion above, if the power demand signal is lower thanthe actual FCPM power, the EESS 14 is charged by the FCPM 12 or byregenerative power of the ETS as long as the EESS 14 stays within itsdefined range. If the EESS 14 is not capable of taking the excess powerfrom the FCPM 12 to be charged, the FCPM power is reduced as much asnecessary to keep within the limit of the EESS 14.

If the power demand signal is higher than the actual FCPM power, theEESS 14 is discharged as long as the EESS 14 stays within its definedranges (SOC, power, etc.). If the EESS 14 is not capable of deliveringthe additional power, the FCPM power is increased as much as necessaryto keep the limit of the EESS 14 and to fulfill the power demand.

The system 10 allows the FCPM 12 to operate smoothly with a changingbase load, i.e., a floating base load. Particularly, fast powertransients of the output of the stack that may occur as a result ofquick and frequent power demands from the vehicle operator are filteredby the average power calculation processor 16 so that these powerdemands are translated more smoothly to the FCPM 12. If the power demandsignal is higher than the FCPM power, then the EESS 14 provides theadditional power as long as the EESS 14 is within its bandwidthtolerance. If the EESS 14 is at its power limit, a higher power requestis made to the FCPM 12.

The smooth power demand transitions provided by the system 10 can bedescribed by way of example. Consider a fuel cell hybrid system that canprovide 150 kW peak power to the ETS, where the maximum power from theFCPM 12 is 100 kW and the maximum power from the EESS 14 is 50 kW. At acertain operating point, the FCPM 12 is providing a base load of 20 kW.The power demand signal goes to 60 kW, and therefore the EESS 14provides the remaining 40 kW. If the power demand signal increases to120 kW, the EESS 14 has to provide its full power capacity of 50 kW andthe FCPM 12 needs to provide 70 kW (50 kW plus the 20 kW base load).

The principle of the invention works as long as the EESS 14 stays withina certain bandwidth of its SOC. If the SOC decreases below apredetermined minimum value, the FCPM 12 needs to provide the requestedpower as long as the requested power is within the power capacity of theFCPM 12. If the power demand signal exceeds the maximum power availablefrom the FCPM 12 when the SOC of the EESS 14 is below its predeterminedminimum value, then the system 10 is unable to provide the requestedpower. It may be desirable in some systems to limit the maximum power tobe equivalent to the maximum power available to the FCPM 12. Therefore,the ETS can be served at any time with the maximum acceleration powerindependent of the SOC of the EESS 14.

In addition, the base load can be controlled depending on the FCPMvoltage. For example, a minimum FCPM power can be defined to avoid acertain upper voltage area, which is known as an operation mode with astrong durability reducing potential. A definition of such a minimumFCPM power or a maximum FCPM voltage is limited by different system andoperation parameters, for example, the idle power consumption of thesystem 10, the energy capacity of the EESS 14, vehicle auxiliary, thedrive cycle, etc.

One further option for the system of the invention is that a certainFCPM base load is already predefined for vehicle start-up. Afterstart-up, the base load is determined as a function of the latest powerrequest that changes the predefined FCPM base load. The time period,which is considered to determine the average power demands, can beadapted to the dynamic limitations of the EESS 14 and the FCPM 12 aswell as to the vehicle performance requirements and the application ofthe hybrid power.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A fuel cell system comprising: a fuel cell power module including afuel cell stack, said fuel cell power module providing a fuel cell powermodule signal indicative of the current output power of the module; anelectrical storage device, said electrical storage device providingpower limit signals indicative of the current power limits of the deviceand a state of charge signal indicative of the current state of chargeof the device; an average power calculation processor responsive to apower demand signal, said average power calculation processor providingan average power signal based on power demand signals from apredetermined previous period of time; a weighting function processorresponsive to the state of charge signal, said weighting functionprocessor providing a weighting signal that decreases as the state ofcharge of the electrical storage device approaches its maximum state ofcharge and increases as the state of charge of the electrical storagedevice approaches its minimum state of charge; a multiplier formultiplying the weighting signal and the average power signal andproviding a multiplied signal; a power comparison processor responsiveto the power demand signal, the fuel cell power module signal and thepower limit signals, said power comparison processor subtracting thefuel cell power module signal from the power demand signal to provide adifference signal between the difference of the current power and thedemanded power, said power comparison processor comparing the differencesignal to the power limit signals of the electrical storage device,wherein if the difference signal exceeds the power limits of theelectrical storage device an output signal is generated to indicate anincrease or decrease of the output power from the fuel cell powermodule, and wherein if the difference signal is within the power limitsof the electrical storage device, the output signal of the powercomparison processor is zero; and an adder responsive to the outputsignal of the power comparison processor and the multiplied signal andproviding an output signal for changing the fuel cell power modulesignal based on the power demand signal.
 2. The system according toclaim 1 wherein the electrical storage device is selected from the groupconsisting of a DC battery, an accumulator, a super-capacitor andcombinations thereof.
 3. The system according to claim 1 wherein thepredetermined previous period of time is a few seconds.
 4. The systemaccording to claim 1 wherein the fuel cell power module recharges theelectrical storage device if the electrical storage device is not at amaximum state of charge.
 5. The system according to claim 1 wherein thepower demand signal comes from an electric traction system of a vehicleand other vehicle loads.
 6. The system according to claim 1 wherein theaverage power calculation processor defines a minimum fuel cell powermodule signal depending on the voltage of the fuel cell power module soas to avoid an upper voltage limit.
 7. The system according to claim 1wherein the power limit signals from the electrical storage deviceinclude an available charge and an available discharge of the electricalstorage device.